1. Field of the Invention
The present invention relates to a VA (vertically aligned)-mode liquid-crystal display device improved in the front contrast ratio.
2. Background Art
These days elevation of the contrast ratio (CR) in liquid-crystal display devices is being promoted. In particular, a VA-mode liquid-crystal display device has the advantage that CR in the normal direction (hereinafter referred to as “front CR”) is high as compared with that in other modes, and various studies and developments are now made for further enhancing the advantage. As a result, in these 6 years, the front CR in VA-mode liquid-crystal display devices has increased from about 400 to about 8000, or by about 20 times.
On the other hand, in liquid-crystal display devices, it is important that not only the front CR is high but also CR in oblique directions (hereinafter this may be referred to as “viewing angle CR”) is high. Various techniques of using a retardation film have been proposed for reducing the light leakage in oblique directions at the time of black level of display in VA-mode liquid-crystal display devices (for example, JP-A 2006-184640). In general, a retardation film is disposed on both the front side and the rear side of the liquid-crystal cell existing in the center therebetween, in which the two retardation films share the retardation necessary for optical compensation in the display device. In general, two systems are employed for the combination for optical compensation. In one system, the retardation films each separately disposed on the front side and on the rear side equally share the same retardation; and the advantage of the system is that the films of the same type can be used therein. In the other system, the retardation film disposed on either one side is made to share a larger retardation; and the system is advantageous in point of the cost since it enables optical compensation by the use of a combination of inexpensive retardation films. In the latter system, in general, the retardation film to be disposed on the rear side is made to share a larger retardation in practical use. One reason is the production cost. Regarding this reason, JP-A-2006-241293 recites as follows in the paragraph [0265]; “In case where the cellulose acylate film of the invention is used only as the protective film of one polarizer (between the liquid-crystal cell and the polarizing film), this may be on either side of the upper polarizer (viewers' side) or the lower polarizer (backlight side) with no functional problem. However, when it is used on the side of the upper polarizer, the functional film must be provided on the viewers' side (upper side) and the producibility may be thereby lowered, and therefore, it may be used on the side of the lower polarizer in many cases, and this may be a more preferred embodiment.” The second reason is that disposing the film having a larger retardation on the rear side is preferred from the viewpoint of the impact resistance and the resistance to environmental change including temperature change and humidity change.
Heretofore, nothing has been investigated about the relationship between the optical properties and the front CR of the retardation film used for improving the viewing angle contrast ratio.